AC Household Power Meter

[ruko]

Hello
I understand non electronic power meters, the ones with a spinning disk, can and do run backward if fed power from a solar array or some other home source. Since the current is alternating through the meter no matter the source, flowing first one way then the other, how does the meter differentiate between the mains and the home source and run backward?
Thanks
ruko

[dmills]

They have two coils, and the torque driving the disk is the product of the two magnetic fields.
One coil in connected in series with the line to sense current and the other across the line to sense voltage...

If you think about it, with the power flowing one way voltage * current is positive on average and for power flowing the other way voltage * current is negative on average... It is a gloriously elegant bit of applied physics, that is actually surprisingly difficult to emulate well with a purely electronic solution.

Regards, Dan.

[soldar]

Since the current is alternating through the meter no matter the source, flowing first one way then the other, how does the meter differentiate between the mains and the home source and run backward?

When the energy is flowing into the house the current is in phase with the voltage. When the power is flowing in the opposite direction then the current is in opposite phase. Logically the meter will spin in the opposite direction. Note though that most meters have a ratchet which prevents backwards spinning.

[ruko]

 Quote: "power flowing the other way" Isn't current flowing in both directions with AC? Still confused.
Thanks

[IDEngineer]

I'll openly admit: I have a similar mental block regarding simple VSWR measuring circuits. They accomplish this in a low-precision way even in very inexpensive ham radios with a couple of diodes, yet I have yet to wrap my head around how it works since it's obviously AC that is being generated and sent to the antenna. I understand the RF coupling, and that each diode rectifies the inductively coupled current in its associated direction, but since everything is AC it would seem that (at least for a CW signal, to keep it simple) the two resulting signals would always be equal regardless of mismatch.

I suppose it could be a summation thing, where the reflected portion sums with the forward portion, but then performance would be phase sensitive depending upon coax length and propagation velocity. I've never seen any mention of such SWR circuits being frequency dependent, despite that coax runs can be anything from almost zero (handhelds and mobiles) to hundreds of feet (tower mounted antennas). Depending upon the frequency, such lengths can be significant modulo percentages of the wavelength.

The spinning disk power meter doesn't phase me at all (pun intended!). But I can't quite grok how SWR can be measured with two diodes on a signal that is zero-centered AC. Everyone has a blind spot, and this is mine.

[dmills]

Reflectometer bridges are basically the same thing as your power meter....

This is possibly most easily seen in the classic low frequency two transformer designs where you have a current transformer and a voltage transformer wired such that in one direction the voltages developed sum while in the other direction they cancel.

Consider if you had a current sensor that output say 1V @ 1A, and a voltage sensor that output say 1V for 50V of input (Both being AC capable devices). Now connect them in series, with 50W  in a 50R circuit you will get ( in one direction) 1V (50V voltage detector) + 1V (1A of current) = 2V = 50W. In the other direction you get 1V (voltage is the same) + -1A (Current is going the other way!) = 0V. Note the fairly spectacular non linearity because we are adding not multiplying but it is good enough in a more or less known impedance circuit when driving a needle on a scale that can be diddled to compensate. 

Do not be confused by the diodes, they just form an RF detector, you could if you wanted to use any other deterctor, polarity is not important, they just detect the amplitude of the RF voltage at the relevant points.

The fact that a simple VSWR bridge does addition not multiplication explains the rather weird scaling and the fact that you have to dial forward power to the reference point to get meaningful readings.

The coupled stripline bridges are sort of the same but you need more theory.

Regards, Dan.

[soldar]

Quote: "power flowing the other way" Isn't current flowing in both directions with AC? Still confused.

You need to study and understand some basic concepts.  Current is not power.  Power is voltage times current.

A battery of 1 volt giving one amp through a 1 ohm resistor is giving one watt.

If you reverse the battery now it is giving minus one amp at minus one volt which multiplied still result in one watt.

If you take the battery and, with a charger, force one amp through it then the battery is receiving power because it is providing minus one amp at plus one volt so it is delivering minus one watt. In other words, the power is flowing from the charger to the battery.

[coppice]

Its all about the relative phases of the voltage and current waveforms. When the phases are aligned peak power is flowing in one direction. As the phases move apart the power factor falls, until the phase difference reaches +-90 degrees. At this point no power flows. As the phase difference increases from +-90 degrees, power flows in the opposite direction, with an increasing power factor. When the phase difference reaches 180 degrees peak power is flowing in the opposite direction.

[ruko]

Thanks to all. I think I may have an inkling of understanding the basic concept. I'll do more thinking on it.

[spec]

In an electrical circuit you have a  source of voltage and current and you have a load. A good example is a battery and a bulb. The battery is the source and the bulb is the load.

If you then reversed the wires to the battery terminals the current would flow through the bulb in the opposite direction, but the battery would still be the source and the bulb would still be the sink.

Once you have grasped that concept the rest is easy. With the mains supply the source just changes polarity 50 times a second (50Hz) or 60 times a second (60Hz) depending on your country.

[timenutgoblin]

assuming that a house had grid-exportable rooftop solar, and the house was connected to the electricity grid, if a current sense resistor was connected between the grid and the house then it should be possible to determine the direction of net power flow by measuring the AC voltages at each end of the current sense resistor wrt neutral. whichever end of the current sense resistor had the higher AC voltage should determine the source of net power flowing through the house. if the AC voltages on each end of the current sense resistor were equal then the net power flowing through the house would be either balanced (grid = solar) or there was no load connected to the house.